The present invention relates to pumps and particularly to peristaltic pumps in which a fluid is forced through a tube by progressively compressing the tube at spaced apart intervals. Pumps of this general type have found particular utility in the medical field for transferring blood and other fluids between a patient and an extracorporeal device.
While a variety of pumps have been previously used, pumps used in connection with extracorporeal circulation are most commonly of the peristaltic type. Medical procedures which commonly employ peristaltic pumps include open heart surgery for circulating blood between a patient and a heart lung machine, dialysis procedures for transferring blood between a patient and a dialyzer, bypass surgery to remove blood from the open thoracic cavity, and continuous care situations for the pumping of intravenous solutions.
Peristaltic pumps are volumetric pumps in which a linearly moving or rotating member progressively compresses a flexible tube at spaced apart intervals to propel a fluid through the tube. A principal advantage of the peristaltic pump is its simplicity of operation and the absence of contact between the fluid, such as blood, and frictional surfaces, such as internal valves, which can be responsible for a variety of hazards. Instead of directly contacting the rotating member of the pump, the fluid is passed through a chemically inert tube.
Pumps of this type exhibit certain disadvantages which are inherent in the pumps themselves. Current roller pumps, often used for pumping blood, are typically driven by a constant speed motor which draws blood at a substantially constant rate. If a line downstream of the pump becomes occluded, the pump can over pressurize and rupture the downstream vessel. If a line upstream of the pump becomes occluded, the pump can generate dangerously low negative pressures, hemolyze the blood, and empty the tissue vessel of the patient causing a collapse of the vessel and resulting in damage to the tissue at the drainage catheter tip. Roller pumps in general are also inefficient in their operation. Much of the energy which is consumed by the pump is used to deform the thick walls of the flexible tubing, an action which in and of itself does not contribute to output flow. Furthermore, during long term use with a patient, the tubing presently employed with standard roller pumps requires being "walked" or replaced because of the wear induced by the cyclic application of high bending stresses as the rollers occlude the tube.
Pumps can be found in the prior art with regulating devices to control the available pumping volume of the pump. For example, in one peristaltic pump, described in U.S. Pat. No. 3,784,323, the material and thickness of the tube is selected so that there is a predetermined differential pressure between the exterior and interior of the tube. Collapse of the tube wall, limiting the flow rate of blood through the tube, occurs as a function of the pump inlet pressure. Thus, the flow rate through the pump will lessen and the tube will become restricted if the supply of blood decreases. This prevents a collapse of the tissue vessel and possible damage thereto. However, the restricted tubing is only partially collapsed and allows the pump to produce very high negative pressures. Furthermore, when in its collapsed state, high bending stress are induced in the crimped edges of the tubing resulting in a limited useful life.
In another peristaltic pump as described in U.S. Pat. No. 4,515,589, the pump is provided with a pumping element comprised of an outer tubing and an inner tubing. An annular air space, maintained between the inner and outer tubing, is vented to atmosphere. When the hydrostatic head of a blood reservoir is maintained above a given pump head level, the inner tubing will expand and fill with blood. When the reservoir level of blood drops below the pump head level, the inner tube collapses and the pump output stops. However, on "full" collapse of the inner tubing, two passageways remain open as the cross-section of the inner tube assumes a dumbbell or .infin. (infinity symbol) configuration. The inner tube is therefore not "completely" collapsed. Also, the inner tubing repeatedly experiences high bending stresses at its edges as it becomes occluded, both during forced collapse caused by the rollers and during collapse as a result of a loss of pressure, which leads to fatigue and wear in the tubing.
With the above described prior art in mind, it is an object of this invention to provide for an improved peristaltic pump having inherent pressure regulation and in which, the output flow is dependent upon the inlet supply. A peristaltic pump according to this invention exhibits low hemolysis and does not generate negative pressures.
Another object of this invention is to provide for a peristaltic pump having inherent pressure regulation while also being capable of exhibiting controllable suction when desired.
A further object of this invention is to provide for a peristaltic pump which exhibits an increased pumping efficiency and which has increased durability permitting use in long term support situations.
In achieving the above objects, the present invention utilizes a combination of features and provides for a peristaltic pump in which a flexible tubing is acted upon by rollers which progressively compress the tubing at spaced apart locations. The tubing itself is of a shape which is naturally flat and occluded when the pressure within the tubing is equal to or less than ambient pressures. The tubing is positioned in the peristaltic pump such that a flat side of the occluded tube is laid flat along the width of the rollers with the edges of the tubing away from the center of the rollers. An air-tight housing connected to a vacuum source encloses the peristaltic pump and allows for control of the pressure within the housing.
As the rollers are rotated, a peristaltic motion is imparted to the tubing. This motion in turn drives the blood from the inlet of the pump to its outlet. During operation, blood is supplied to the inlet at a pressure above ambient, the tubing fills and inflates, and the progressive compression of the tubing produces the pumping action. With the internal pressure of the housing at ambient, if the supply of blood to the pump is discontinued, blood will be pumped out of the tubing as the tubing completely flattens back into its natural or free condition. Being completely flat and occluded when no blood is being supplied to the pump, no negative pressures are generated within the tubing by the rollers.
If a slight vacuum is applied to the housing, thereby biasing the tubing into an unoccluded state, a suction equal to the vacuum will be produced as the rollers are rotated over the tubing. Thus, the pump can be used as a suction pump during operation.
Additionally, the novel shape of the tubing in the present invention allows the tubing to assume its completely occluded position without inducing high bending stresses along the edges of the tubing. This design therefore minimizes wear and fatigue and increases the durability of the tubing enabling and enhancing use in long term use situations.
As an alternative to the completely occluded tubing and vacuum housing mentioned above, when it is desirable to operate the pump as a suction pump, a slight and controllable suction can be generated within the tubing by employing a resilient tubing which is mildly biased into an unoccluded free cross-sectional shape. In this version, the tubing is provided with a mild unoccluded bias so that when a given pressure, less than ambient, is generated within the tubing, the tubing will collapse into its completely occluded or flat condition. The complete collapse of the tubing at a low negative pressure prevents higher hazardous negative pressures from being generated within the tubing.
The tubing can also be provided with a variable cross-sectional width. A variable cross-section tubing can allow for minimizing of the total pump priming volume and can allow for higher withheld pressures between the rollers at lower tensions of the tubing.
The peristaltic pump of the present invention also includes a mechanism for controlling the positive pressure which can be generated within the tubing. In this regard, the pump is provided with a mechanism which varies the tension of the tubing. The tensioning mechanism simultaneously adjusts the positions of both the inlet and outlet ends of the tubing. This reduces the distance which has previously required, in re-positioning the inlet or outlet alone, for the same amount of tension adjustment. The tensioning mechanism also "absorbs" or counters displacement of the tubing caused by the changing position of the rollers and thus allows for a flexible but inelastic tubing to be used.
One embodiment of a generally inelastic tubing is disclosed. In the embodiment, the tubing is reinforced with a fabric mesh that substantially prevents elongation of the tubing under tension.
An adjustable occlusion block is provided with the pump to permit accurate control of the outlet pressure as a roller disengages from the tubing adjacent to the outlet of the pump.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims taken in conjunction with the accompanying drawings.